Update std::os module documentation.

Adds missing descriptions for the modules std::os::linux::fs and std::os::windows::io.
Also adds punctuation for consistency with other descriptions.

library/std/src/os/linux/fs.rs

@@ -1,3 +1,5 @@
+//! Linux-specific extensions to primitives in the `std::fs` module.
+
 #![stable(feature = "metadata_ext", since = "1.1.0")]
 
 use crate::fs::Metadata;